In recent years, a lighting device using a light-emitting diode (LED) that is semi-permanent in lifespan and is very low in power consumption has been introduced in various manners. LEDs are more stable and reliable that other thermal conversion light-emitting elements. In addition, LEDs are low in power consumption and is long in lifespan. Currently, an LED technology is developing by leaps and bounds, and high-efficiency, high-brightness LEDs having a variety of colors are being released on the market. A single LED acts as a point light source, but a plurality of LEDs collected together forms a linear light or a surface light source so that they can be utilized as a lighting device.
A lighting using LEDs is, first of all, excellent in energy-saving effect due to low power consumption. In addition, the lighting using LEDs has an effect of reducing the environmental pollution by substituting for a conventional electric lamp including variety of hazardous substances and having a short lifespan.
The LED lighting includes an LED unit consisting of a plurality of LEDs connected in series. The driving of the LED unit requires that the LED unit should be applied with a voltage greater than the sum of the forward operating voltages of the series connected LEDs. A typical LED driving device rectifies commercial AC power into DC power and converts the rectified DC power into a predetermined magnitude of DC power through a switch mode power supply (hereinafter, referred to as “SMPS”) for application to the LED unit. However, the SMPS generates the predetermined magnitude of DC power through a high-speed switching operation to cause much noise, thus leading to interference, which adversely affects the surrounding circuit elements. In order to compensate for this adverse effect, an LED driving device employing the SMPS entails a problem in that other circuit components including a noise filter and the like must be additionally provided, thus leading to an increase in volume and weight and thus increasing the manufacturing cost.
In an effort to solve the problem involved in the conventional LED driving device employing the SMPS, an AC-direct drive type LED driving device has been developed and used. The AC-direct drive type refers to a method of driving the LEDs by directly applying the rectified DC power to the LED unit, instead of rectifying commercial AC power into DC power and converting the rectified DC power into a predetermined magnitude of DC power through an SMPS for application to the LED unit.
FIG. 1 is a block diagram illustrating one example of a conventional AC-direct drive type LED driving device in accordance with the prior art.
Referring to FIG. 1, a power supply unit 10 supplies commercial AC power, and a rectification unit 20 rectifies the commercial AC power applied thereto from the power supply unit 10 into DC power. Herein, the rectification unit 20 may use various rectifier circuits that rectify AC power into DC power, including a bridge diode rectifier circuit, which falls within the scope of the present invention. The DC power rectified by the rectification unit 20 has a magnitude of 0 V at 0°, a magnitude of 70.7% of the maximum power magnitude at 45°, and the maximum power magnitude of 100% at 90°, respectively.
The rectification unit 20 applies the rectified DC power to an LED unit 40 in which five LEDs are connected in series. A power measurement unit 50 measures the magnitude of the DC power applied to the LED unit 40. In this case, the LED unit 40 may be configured such that a different number of LEDs are connected in series.
A light emission control unit 30 controls the operation of the LEDs such that a maximum number of LEDs connected in series are driven depending on the magnitude of the power applied to the LED unit 40 based on the magnitude of the DC power measured by the power measurement unit 50. More specifically, the operation of the light emission control unit 30 will be described hereinafter. A control signal generation unit 31 controls the operation of switches SW1 to SW5 based on the measured magnitude of the DC power and the operating voltage Vf of the respective LEDs constituting the LED unit 40. In other words, if the measured magnitude V of the DC power is smaller than or equal to 2Vf (i.e., V≦2Vf), the control signal generation unit 31 controls a first switch SW1 to be turned on and the remaining switches SW2, SW3, SW4 and SW5 to be turned off so that only a first LED of the LED unit 40 is controlled to be driven. If the measured magnitude V of the DC power increases gradually and then is larger than or equal to 2Vf and 3Vf (i.e., 2Vf≦V≧3Vf), the control signal generation unit 31 controls a second switch SW2 to be turned on and the remaining switches SW1, SW3, SW4 and SW5 to be turned off so that the first LED and the next LED of the LED unit 40 are controlled to be driven sequentially in order. If the measured magnitude V of the DC power is larger than or equal to 5Vf (i.e., V≧5Vf) in the same manner as in the above case, the control signal generation unit 31 controls all the switches SW1 to SW5 to be turned off so that all the LEDs constituting the LED unit 40 are controlled to be driven sequentially in the serial connection order (L1→L2→L3→L4→L5→L6) thereof.